The invention relates to a product that can be exposed to a hot aggressive gas, with a metallic basic body provided with a bond coat forming a bonding oxide and a ceramic thermal barrier coating. The invention furthermore relates to components that can be subjected to a hot gas in thermal machines, particularly in a gas turbine, which are provided with a thermal barrier coating to protect them against a hot aggressive gas.
U.S. Pat. No. 4,585,481 discloses a protective coating to protect a metallic substrate made of a superalloy against high-temperate oxidation and corrosion. A MCrAlY alloy is used for these protective coatings. This protective coating has 5% to 40% chromium, 8% to 35% aluminum, 0.1% to 2% of an oxygen active element selected from Group IIIb of the periodic system, including the lanthanides and actinides and mixtures thereof, 0.1% to 7% silicon, 0.1% to 3% hafnium, and a balance comprising nickel and/or cobalt (the percentages indicated are weight percent). The corresponding MCrAlY alloy protective coatings according to U.S. Pat. No. 4,585,481 are applied by plasma spraying.
U.S. Pat. No. 4,321,310 describes a gas turbine component with a basic body made of a nickel-based MAR-M-200 superalloy. A MCrAlY alloy layer is applied to the base material, particularly a NiCOCrAlY alloy with 18% chromium, 23% cobalt, 12.5% GB 745 257 A discloses a process for coating a metal or another material with stable metal oxides. The other materials indicated, which may be used as a substrate for a coating, are ceramic materials and graphite. Various spinels, e.g., chromite FeOxe2x97xafCr2O3, chrysoberyl BeOxe2x97xafAl2O3, gahnite ZuAl2O4, geikielite (Mg, Fe) Oxe2x97xafTiO2 and MgOxe2x97xafAl2O3 (aluminate spinel) are applied as the coating material to the substrate by means of thermal spraying. With this process, the aforementioned minerals are sprayed, for example, onto the turbine blades of aircraft engines. aluminum, 0.3% yttrium and a balance of nickel. This MCrAlY alloy layer has a polished surface to which an aluminum oxide layer is applied. A ceramic barrier coating with a columnar structure is applied to this aluminum oxide layer. Due to this columnar microstructure of the thermal barrier coating, the crystallite columns are perpendicular to the surface of the basic body. The ceramic material specified is stabilized zirconium oxide.
U.S. Pat. No. 5,236,787 discloses the insertion of an interlayer of a metal-ceramic mixture between the basic body and a ceramic thermal barrier coating. This is intended to cause the metallic proportion of this interlayer to increase toward the basic body and to decrease toward the thermal barrier coating. Conversely, the ceramic proportion is to be low near the basic body and high near the thermal barrier coating. The thermal barrier coating specified is a zirconium oxide stabilized with yttrium oxide with components of cerium oxide. With this interlayer an adaptation oft the different thermal expansion coefficients between the metallic basic body and the ceramic thermal barrier coating is to be achieved.
EP 0 486 489 B1 discloses a corrosion resistant protective coating for intermediate and high temperatures of up to approximately 1050xc2x0 C. for a gas turbine component made of a nickel-base or cobalt-base alloy. The protective coating has (in percent by weight) 25% to 40% nickel, 28% to 32% chromium, 7% to 9% aluminum, 1% to 2% silicon, and 0.3% to 1% of at least one reactive rare earth element, at least 5% cobalt, and optionally 0% to 15% of at least one of the elements of the group consisting of rhenium, platinum, palladium, zirconium, manganese, tungsten, titanium, molybdenum, niobium, iron, hafnium and tantalum. In a concrete embodiment, the protective coating contains the elements nickel, chromium, aluminum, silicon, yttrium, and rhenium in a range of 1% to 15% and a balance of cobalt. The addition of rhenium clearly enhances the corrosion protective properties.
WO 96/34128 A1 discloses a product, particularly a gas turbine blade, with a metallic substrate. A protective coating system comprising a bond coat and a thermal barrier coating is applied to the metallic substrate. The thermal barrier coating consists of a columnar ceramic oxide, particularly made of a partially stabilized zirconium oxide. This thermal barrier coating is bonded to the metallic substrate via an anchoring layer. The anchoring layer in turn is bonded via the bond coat to the metallic substrate, particularly a nickel-based or cobalt-based superalloy. The bond coat consists of a MCrAlY alloy, such as indicated, for example, in U.S. Pat. Nos. 5,154,885; 5,268,238; 5,273,712, and 5,401,307. The anchoring layer for its part consists of a spinel comprising aluminum and an other metallic element. The other metallic element is preferably zirconium. The anchoring layer is preferably applied by means of a PVD process, particularly an electron beam PVD process, in an oxygen-containing atmosphere. During the coating operation, the metallic substrate is kept at a temperature of above 700xc2x0 C. The thickness of the anchoring layer is preferably less than 25 xcexcm.
WO 96/31293 A1 describes a protective coating system for a gas turbine blade that is applied to a superalloy component for protection. The protective coating system comprises a zirconium oxide-based thermal barrier coating. To this zirconium oxide-based thermal barrier coating, a wear coat is applied that is to prevent premature damage to the thermal barrier coating. Such premature wear of the unprotected thermal barrier coating occurs due to contact with a hot aggressive gas containing oxides of calcium or magnesium. The wear layer has a composition that reacts with the oxides in the hot aggressive gas, which increases the melting temperature and the viscosity of the wear layer. For this purpose, the wear layer comprises, for example, aluminum oxide, magnesium oxide, chromium oxide and a spinel, e.g., magnesium-aluminum oxide.
U.S. Pat. No. 5,466,280 (corresponding to GB 2 286 977 A1) discloses a composition for an inorganic coating applied to a low alloy steel and resistant to high temperatures. The predominant property of the coating is that it provides increased corrosion resistance by incorporating iron into the coating. The coating is created by converting different metal oxides, such as magnesium oxide, aluminum oxide, iron oxide and calcium oxide at temperatures of above 1000xc2x0 C. into spinels, which are not further specified.
German Application 15 83 971 discloses a refractory protective layer for metallurgical furnaces, which protective layer has a spine), namely MgOxe2x80x94Al2O3. German Patent 37 37 215 discloses a protective coating containing spinel (MgOxe2x80x94Al2O3) for an electrochemical sensor to determine the oxygen content in gases, particularly exhaust gases of internal combustion engines of automobiles.
EP 0 684 322 A2 discloses a MgOxe2x80x94SiO2 and/or MgOxe2x80x94Al2O3 based ceramic coating made particularly of forsterite (Mg2SiO4), spinel (MgAl2O4) or cordierites (2MgO-2Al2O3-5SiO3).
The object of the invention is to define a product, particularly a component of a gas turbine, with a metallic basic body and a thermal barrier coating disposed thereon.
The invention is based on the finding that currently used ceramic thermal barrier coatings, despite the use of, e.g., partially stabilized zirconium oxide, have a thermal expansion coefficient which at maximum is only about 70% of the thermal expansion coefficient of the metallic basic body used, particularly of a superalloy. This lower thermal expansion coefficient of the zirconium oxide thermal barrier coatings compared to the metallic basic body causes thermal stresses during exposure to a hot gas. To counteract such resultant stresses occurring under alternating thermal stress, an expansion-tolerant microstructure of the thermal barrier coating is required, e.g., by adjusting a corresponding porosity or a columnar structure of the thermal barrier coating.
The use of a spinel provides a thermal barrier coating, possibly taking into account mixed crystal formation and microstructure modification, with a high thermal expansion coefficient, low thermal conductivity, a high melting point, high chemical stability, a reduced tendency toward sintering, and a high phase stability.
According to the invention, the product-related object is attained in that the thermal barrier coating has a spinel of the composition AB2X4, where X represents oxygen, sulfur, selenium and/or tellurium. A represents an element or several elements of the group comprising aluminum, magnesium, manganese, iron, cobalt, nickel, copper, zinc, cadmium, silicon, titanium and tungsten. B represents one or several elements of the group comprising aluminum, magnesium, manganese, iron, vanadium, chromium, gallium, silicon, titanium, sodium and potassium.
The thermal barrier coating is bonded to the basic body either directly or indirectly via a bond coat. Bonding is preferably effected via an oxide layer that is formed, for example, by oxidation of the basic body or of the bond coat. Bonding can also or additionally be effected through mechanical anchoring, e.g., through roughness of the basic body or the bond coat.
Such a thermal barrier coating particularly serves to prolong the life of products that are subjected to a hot gas, e.g. gas turbine components, blades and heat shields. It exhibits low thermal conductivity, a high melting point, and is chemically inert.